Spontaneous potential curves, sometimes referred to as self potential curves, record the electrical potential (voltage) produced by the interaction of formation connate water, conductive drilling fluid, and certain ion-selective rocks. It is a basic component of almost all formation evaluation work. Such curves are useful for: differentiating potentially porous and permeable reservoir formations, such as sandstone, limestone, and dolomite, from nonpermeable formations such as clays and shales; defining bed boundaries and permitting correlation of beds; giving a qualitative indication of bed shaliness; aiding in mineral identification; and permitting the determination of formation water resistivity.
The curve generated by conventional methods is usually a recording versus depth of the difference between the electrical potential of a movable electrode in the borehole and the electrical potential of a fixed surface electrode. A description of such technology can be found in "Fundamentals of Well-Log Interpretation, Part I, The Acquisition of Logging Data" by O. Serra, Elsevier, New York, 1984, at pages 77 to 88. A typical conventional method is one illustrated in (a) of FIG. 1 hereof wherein a wireline apparatus is shown which includes: two electrodes, one at the surface (N) and the other in the wireline apparatus in the borehole (M); a high impedance millivoltmeter (MV), which is comprised of a galvanometer G and resistor R, and an adjustable counter-potential (A) to offset the various polarization or extraneous DC potentials which may occur. Such conventional methods have several disadvantages. For example, the measurement is made between two electrodes which can be from about 500 to about 30,000 feet apart, thus being susceptible to spurious potentials which are frequently induced and which can alter the accuracy of the curve. Furthermore, the electrodes are usually made of lead or steel, the surface of which are susceptible to oxidation. A slow varying polarization may occur creating drifts in the recording. Also, magnetization of the spool of cable can also induce a slow sinusoidal varying potential. If the apparatus is computerized, the simplicity of conventional methods may be a drawback since quantifiable parameters must be used to correct for all of the spurious effects mentioned above. By the practice of the present invention, a spontaneous potential curve can be generated by a wireline test tool without encountering many of these problems.
Logging while drilling is becoming more and more common owing to improved technology and the advantages of getting formation evaluation data in real-time. Conventional techniques make it impossible to obtain a spontaneous potential curve while drilling, primarily because no insulated electrical connector exists between the surface and downhole. Thus, there remains a substantial need in the art for new methods and apparatus which are capable of recording self potential curves while drilling.